The present invention relates to an electron beam lithography system for delineating a desired pattern on a target by means of an electron beam, and more specifically to an electron beam lithography system which can maintain its exposure accuracy despite a proximity effect.
Various electron beam lithography systems have recently been developed which delineate a fine pattern on the surface of a target, such as a semiconductor wafer, mask substrate etc., by means of an electron beam. Since patterns have become finer and finer, electron beam lithography systems need to maintain exposure accuracy in spite of a proximity effect resulting from the electrons scattered in the substrate and in a resist film formed on the surface of the substrate, and from other causes.
Methods of correction of the proximity effect applicable to the electron beam lithography systems include:
(1) To correct the dose of electron beams applied to the substrate; PA1 (2) To correct the shape of the pattern formed on the substrate; and PA1 (3) To form a multilayer resist film on the substrate.
If method (1) is applied to the lithography systems, massive calculation must be performed to obtain the corrected value. Moreover, method (1) cannot be applied to lithography systems of a raster-scanning type. If method (2) is applied, the shapes of pattern must be subjected to a very fine adjustment. Method (3) complicates the processes of coating a resist film and developing patterns.
Thereupon, a fourth method has been proposed. In this method, a background region of the substrate is exposed by a defocused electron beam whose beam current is lower than that of the electron beam for delineating the pattern. This method helps to maintain dimensions of the pattern, despite the electrons scattered from the substrate.
In the prior art electron beam lithography systems, if an electron beam is incident on the resist film formed on the substrate surface, the energy absorbed by the resist film consists of a first component produced by forward scattered electrons and a second component produced by backwardly scattered electrons. The forwardly scattered electrons include the electrons incident on the resist film and the electrons scattered within the resist film. The backwardly scattered electrons are ones scattered back from the substrate. The first component spread within a range of 0.1 to 0.2 micron, and the second component within a range of 1 to 10 microns. These ranges change according to the acceleration of the electron beam. After a pattern has been delineated, the spread of the second component influences the quantity of energy absorbed by a certain portion of the resist film. Due to the spread of the second component, the pattern formed on the substrate cannot have the desired dimensions.
In the fourth method, the background is repeatedly exposed by a defocused electron beam which provides the same absorbed energy quantity distribution as does the second component. Therefore the sum of the second component of the electron beam and a component of the defocused electron beam is uniformly distributed on the surface of the substrate. Thus, the influence of the second component on the pattern dimensions is reduced. Since the spread of the first component can fully be suppressed to 0.1 micron or less by, for example, increasing the acceleration voltage, the fourth method can reduce the proximity effect can, therefore, provide an accurate pattern.
If the fourth method is applied to the lithography systems in practical use, however, the pattern must be first delineated by an electron beam and the background must be then exposed by another electron beam defocused for correction. This would reduce the exposure throughput to half or less.